Monoclonal antibodies (Mabs) to whole M. hominis strain 1620 were raised in order to attempt to characterize surface molecules involved in attachment. Strain 1620 was the first of many isolates obtained over six years from the synovial fluid of a chronically infected septic arthritis patient. We obtained three Mabs that do not cross react with any other human species. The Mabs were shown to be specific for surface exposed, integral membrane proteins that appear to be acetylated. We used these monoclonals to screen 15 other strains of M. hominis to look for similarities in surface protein expression. Although the mabs bind to a discrete number of surface proteins, the proteins expressed by any given strain varies, illustrating once again the heterogeneity among isolates of this species. Subsequently, we probed the 14 sequential isolates from the septic arthritis patient. Although the restriction patterns of the DNAs, DNA hybridization patterns to the EF-Tu gene and rRNA operons, as well as the DNA methylation patterns, were identical, the proteins expressed on the surfaces of these isolates varied with time and perhaps drug therapy, suggesting that antigenic variation is a mechanism that can be used by M. hominis to evade the host immune system.